Vaillant boiler not responding to request for heat

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Got a strange issue where the boiler is stuck in state of s.30 which is no demand. Initially tested by looping pin 3 and 4 together and boiler fires up no problem for water and heating.

Assumed this was a fault with the thermostat so replaced with a new one. However boiler still not responding to any demand calls from thermostat. Thinking it could be the boiler PCB at fault but then it wouldn't have fired up when looping pins 3 and 4.

Only thing I can think is the Cables damaged somehow and just needs testing for continuity. Any other ideas?

Just to note no changes have occurred to the system before the issue and no fault codes have come up. Vaillant ecotech plus 612, 10 years old.
 
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You could link the wires at the thermostat, if it fires then you have another dodgy stat, if it doesn't then sounds like the cable, and if that's the case, a wireless stat would be your best bet, maybe go down the internet enabled route-Hive, Nest etc.
 
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The 612 boiler is a system boiler, so there will be a 3 port valve or at least 2 2 port valves somewhere. It/ they may have failed, or the timer could be faulty.
If when you linked 3 and 4 on the PCB the boiler fired up then the PCB is unlikely to be faulty.
 
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Figured it out. Isolator switch had been hidden behind a big cabinet and must have got knocked somehow. Real face palm moment :LOL::LOL:
 
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